SUMNER: KTJPFFER'S VESICLE 59 



III. RELATION OF PROSTOMA TO GERM LAYERS. 



Prostomal Thickening and Gut-Hypoblast. The exact part played by the 

 prostomal cells in the formation of the germ-layers is very difficult to determine and 

 in spite of a painstaking study upon a great many embryos, I cannot regard my 

 observations on this point as quite conclusive. As already shown (figures 6 and 

 13) in sagittal sections at a certain period, a direct continuity is observable between 

 what I have called the " prostomal thickening " and a thin layer of cells which evi- 

 dently represents the gut-hypoblast. This continuity is strong evidence that the 

 latter has been derived by proliferation from the former. In the trout the pros- 

 tomal invagination commences before the forma- 

 tion of the germ ring. In Mursenaf on the other FIGURE is. 

 hand, at a time when the blastoderm has covered 

 nearly one-half the circumference of the yolk, 

 neither prostomal invagination nor gut-hypoblast 

 are to be seen, although the germ-ring is well 



Transverse section passing through posterior 



advanced. At the next Stage which I have Sec- eud of a very early trout emhryo (somewhat 

 ,. j ,n i n\ ii 11,- later than that in figure 6). PrHy, hypoblast 



tioned (figure 13) the gut-hypoblast is seen to be invaginated at ,, pro ; toma . 



in direct continuity with the invaginated cells, 



which agree witli the former in showing a higher staining power than the rest of 



the blastoderm. 



Transverse sections of these same stages unfortunately do not show these condi- 

 tions with the same degree of clearness. 1 The lowermost cells do not exhibit the dis- 

 tinct epithelial arrangement seen in longitudinal sections. In fact what appears in 

 the latter as a continuous ventral layer, extending beneath the entire embryonic 

 region of the blastoderm, is only to be observed with distinctness in those cross-sec- 

 tions which pass through the extreme posterior end. It is clearly marked in several 

 blastoderms which I have cut (figure 18). What becomes of this layer in the cepha- 

 lad portion is not revealed in transverse sections of embryos at such an early stage. 

 At a later period when the gut-hypoblast has finally become distinctly differentiated 

 throughout the entire region of the embryo, it is found that a conspicuous break in 

 its continuity occurs along a considerable extent of the axial line. The gut-hypo- 

 blast now occurs in two lateral sheets, each sheet being merged axiad into the un- 

 differentiated cord of cells which form the " primitive streak " (figure 19). 



The question at once arises was the lowermost germ-layer, at the time of its first 



1 I have transverse sections of Nolurus, Amiurus and Salvelinun embryos during this period, but unfortunately, none 

 of Slnrtsna? 



